There is a step-down type DC-DC regulator (buck regulator) in which an output (direct-current) voltage lower than an input (direct-current) voltage is output to a load as a power-supply circuit.
FIG. 8 is a view illustrating a configuration example of a general step-down type DC-DC regulator. In FIG. 8, a reference numeral 101 is a first transistor, a reference numeral 102 is a second transistor, a reference numeral 103 is an inductor, a reference numeral 104 is a capacitor, a reference numeral 105 is a load, a reference numeral 106 is a schottky barrier diode (SBD), a reference numeral 107 is a first control circuit, a reference numeral 108 is a second control circuit, and a reference numeral 109 is a voltage source.
The first transistor 101 is a high voltage side (high-side) switching element, and the second transistor 102 is a low voltage side (low-side) switching element. In the first transistor 101, a drain is coupled to a power supply line to which an input voltage from the voltage source 109 is supplied, and a source is coupled to one end of the inductor 103. In the second transistor 102, a drain is coupled to an interconnection point between the source of the transistor 101 and the one end of the inductor 103, and a source is coupled to a reference voltage line (for example, a ground).
The first transistor 101 and the second transistor 102 are on/off controlled by rectangular waves (pulse signals) respectively output from the first control circuit 107 and the second control circuit 108. The DC-DC regulator illustrated in FIG. 8 outputs an output voltage to the load 105 via a low-pass filter made up of the inductor 103 and the capacitor 104 by alternately turning on/off the transistors 101, 102 with the rectangular waves output from the control circuits 107, 108.
Here, when the transistors 101, 102 are simultaneously turned on in the DC-DC regulator illustrated in FIG. 8, a short circuit occurs between the power supply line and the reference voltage line, a short-through current flows through the transistors 101, 102, and heat is generated and so on in circuit elements. A period called as a dead time is therefore provided in the DC-DC regulator as illustrated in FIG. 8 to prevent that the transistors 101, 102 are simultaneously turned on. The dead time is the period when the transistors 101, 102 are simultaneously turned into off states by shifting a timing to turn-off one transistor and a timing to turn-on the other transistor.
The DC-DC regulator is in a state in which it does not operate as a circuit because both of the transistors 101, 102 are in the off states during the dead time, but efficiency deteriorates because loss is generated during the dead time. There is a method to keep current flow through a path of the inductor 103, the load 105, the schottky barrier diode 106 during the dead time by providing the schottky barrier diode 106 to make a return current flow as illustrated in FIG. 8 to improve the efficiency deterioration. Note that a method is also conceivable in which the schottky barrier diode 106 is not provided but a body diode of the second transistor 102 is used. However, an effect of the efficiency improvement is small because a reverse recovery time (a time to be an off state from a continuity state) is long and a forward drop voltage is also large in the body diode compared to the schottky barrier diode.
A DC-DC converter enabling an electro static discharge protection function and a conversion efficiency improvement at a low current area is disclosed in Patent Document 1. Besides, a method is disclosed in Patent Documents 2, 3 in which the dead time is made short by adjusting a period from the time one transistor is turned off to the time the other transistor is turned on based on a voltage detection result and so on at a detection point in a step-down type DC-DC converter.
In the step-down type DC-DC regulator as illustrated in FIG. 8, it is desirable that the return current is made flow by using the schottky barrier diode of which reverse recovery time is short to suppress the efficiency deterioration caused by the dead time. However, there are problems in which the schottky barrier diode is expensive, and a high withstand voltage product is few.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2010-27709
[Patent Document 2] Japanese National Publication of International Patent Application No. 2006-508629
[Patent Document 3] Japanese Laid-open Patent Publication No. 2007-185050